The present invention relates to a method of manufacturing a semiconductor device, and particularly to a method of improving a burying characteristic in burying a recessed portion formed in an insulating film with a conductive film by high pressure reflow.
Recent VLSIs are required to integrate millions of elements on a chip of several mm square. Such a VLSI is essential to adopt a multilayer interconnection structure for suppressing an increase in area ratio of an interconnecting portion on the basis of the total chip area. The thickness of an interlayer insulating film for insulating upper and lower interconnections from each other, however, cannot be thinned more than a specified value for ensuring the insulating performance. This makes it difficult to reduce the design rule of a substrate in the vertical direction as compared with that in the horizontal direction. Moreover, from now on, such an interlayer insulating film tends to be planarized by chemical/mechanical polishing (CMP) or the like for ensuring the resolution of lithography and the reliability of an upper interconnection, and thereby a local difference in thickness is increased for the interlayer insulating film on a substrate. This tends to increase the aspect ratio of a connection hole opened in the interlayer insulating film. For example, there appears a connection hole having an aspect ratio increased up to a value of from 4 to 5.
To cope with the increased aspect ratio of a connection hole, it has come to be important to bury the connection hole with a conductive (metal) film, and various metal burying techniques such as a brancket W (tungsten)-CVD method, high temperature Al sputtering method, and Al reflow method have been examined, and partially put in practice.
The blanket W-CVD method for forming a W film over the entire surface of a substrate has been most extensively used at present from the viewpoints of an excellent burying characteristic and a high process stability. Such a technique, however, is practically used only for a plug portion (buried in a connection hole) because a Ti based adhesive layer must be formed for enhancing adhesiveness of the W film to an insulating film and the resistivity of W is higher than Al. Accordingly, this technique has various problems that an etch-back process of the W film is required; the terminal determination of the etch-back is difficult; an interconnection extracting portion to be connected to the plug portion must be formed on the interlayer insulating film by patterning of an Al film; and WF.sub.6 as a source gas for the W film is expensive. These factors increase the number of processes and complicate the processes, to thereby reduce the throughput and increase the cost.
On the contrary, the high temperature Al sputtering method and the Al reflow method are advantageous in that the number of processes is smaller than the bracket W-CVD method because the plug portion and the interconnection extracting portion can be simultaneously formed by a single kind of metal. The high temperature Al sputtering method is intended to promote the surface migration of Al atoms deposited on a substrate by keeping the substrate at a high temperature during sputtering, thereby improving the burring characteristic in-situ. On the other hand, the Al reflow method is intended to form an Al film on a substrate by a usual method such as sputtering and to heat the substrate at a temperature in a range of from a recrystallization temperature of Al to a melting point (about 660.degree. C.) of Al, thereby allowing the Al film to reflow on the substrate for burying. Each of these techniques, however, is required to heat a substrate at a temperature more than about 500-550.degree. C. for burying, which tends to exert adverse effect on the already formed Al interconnection, to generate an Al spike on the Si substrate, or to degrade the surface planarization of the Al film itself. The burying abilities of these techniques are also low. For example, these techniques are limited to be applied to a hole having an aspect ratio less than about 2 to 3, and consequently, they are impossible to cope with the future high aspect ratio.
In view of the foregoing, a high pressure reflow method modified from the above-described Al reflow method has been proposed. This method is intended to perform-the reflow in an inert gas atmosphere at a pressure in a range of about from several tens to one hundred and several tens atm as described in "Abstracts of IEDM, pp. 105-108, 1994". This method will be described below with reference to FIG. 15.
Referring to FIG. 15, a viahole 33 is opened in a SiO.sub.x interlayer insulating film 32 covering a lower interconnection 31 in such a manner as to reach the lower interconnection 31. A Ti underlying film 34 is formed in such a manner as to cover on the inner surface of the viahole 33, and an Al--Cu conductive film 35 containing Cu in an amount of from 0.5 to 2% is formed in such a manner as to block the opening end of the viahole 33. Here, the Ti film 34, which is provided for enhancing adhesiveness between the SiO.sub.x interlayer insulating film 32 and the Al--Cu film 35, is typically formed by sputtering (hereinafter, referred to as "sputtering film").
The Al--Cu film 35 is also typically formed by sputtering. The sputtering, however, is originally poor in step coverage. In particular, when the opening diameter of the viahole 33 is fine at the sub-half micron level and the aspect ratio of the viahole is large, sputter particles are difficult to reach the bottom of the hole; and the deposited film becomes thick while forming an overhung near the opening end of the hole and then the leading ends of the overhung portions are joined to each other to block the opening end. A void 36 shown in FIG. 15 is formed by such a mechanism. The deposition profile on the wafer, however, is extremely advantageous for the high pressure reflow method intended to press the conductive film in the connection hole while applying a high pressure to the conductive film in an inert atmosphere such as Ar. With such a high pressure reflow method, the viahole 33 can be perfectly buried with the Al--Cu film 35 in the suitable condition, and the surface of the Al--Cu film 35 is planarized. At this time, an inert gas remaining in the void 36 seems to be absorbed by the Al--Cu film 35.
In addition, the high pressure reflow is performed at a substrate temperature of from 400 to 450.degree. C., which is lower than that (about 500 to 550.degree. C.) of the high temperature Al sputtering or a usual Al reflow. This is advantageous in preventing connection breakage of a contact portion with the Si substrate, preventing adverse effect exerted on an already formed Al interconnection, and preventing surface roughness of the Al--Cu film itself. The high pressure reflow-method can also cope with a connection hole having an aspect ratio in a range of about from 4 to 5.
Incidentally, the detail mechanism of burying a connection hole by the high pressure reflow method is not perfectly apparent; however, it is known by experience that the growth of a hard metal oxide having a high melting point on the surface of a conductive film obstructs the thermal flow of the conductive film to suppress the smooth burying. Such a metal oxide may be formed on the upper surface of a conductive film or at the boundary with an underlying film depending on the kinds of the multilayer structure, forming method and forming system of the conductive film. In each case, the metal oxide film degrades the burying characteristic. The formation of such a metal oxide film will be described with reference to FIGS. 16 and 17.
FIG. 16 shows a state that an Al oxide film 37 is grown on the surface of the Al--Cu film 35 in FIG. 15. The sputtering system used for forming the Al--Cu film 35 is usually independent from the high pressure reflow system for allowing the Al--Cu film to reflow at a high pressure, and thereby the wafer completed in sputtering is carried from the sputtering system once to atmospheric air and is carried in the high pressure reflow system. The Al oxide film 37 is formed during the wafer is carried in atmospheric air. When the viahole 33 is intended to be buried with the Al--Cu, film 35 in such a state by high pressure reflow, the burying does not proceed over a specified level, and consequently, the void 28 remains as shown in FIG. 16.
In some cases, the underlying film is first oxidized depending on the film quality of the interlayer insulating film, and the conductive film is also oxidized through the oxidation of the underlying film. Such a phenomenon is particularly generated when the step coverage is poor upon formation of the underlying film and the thickness of the underlying film on the side wall surface of the connection hole is thin. Specifically, as shown in FIG. 17, when the thickness of the Ti film 34 on the side wall surface of the viahole 33 is extremely thin and the SiO.sub.x interlayer insulating film 32 contains a large amount of OH groups due to the forming method, moisture is discharged from the SiO.sub.x interlayer insulating film 32 by heating of the substrate upon high pressure reflow, so that the Ti film 32 is converted into the Ti oxide film 39 due to the discharged moisture. Thus, the portion of the Al--Cu film being in contact with the Ti oxide film 39 is oxidized, and consequently an Al oxide film 38 is formed at the (Al--Cu)/Ti boundary. In this way, the Al oxide film 38 present at the boundary with the underlying film also exerts adverse effect on the burying characteristic.
As one of measures for solving such a problem, there has been proposed a multi-chamber system capable of continuously performing a series of processes from the sputtering of an underlying film to high pressure reflow of a conductive film without exposure of a wafer to atmospheric air. Such a system has a configuration shown in FIG. 18, wherein a load lock chamber 202 for containing a wafer cassette, Ar.sup.+ sputtering/etching chamber 203, Ti sputtering chamber 204, Al sputtering chamber 205, high pressure reflow chamber 206 are connected to respective sides of a carrying chamber 201 formed in a polygonal shape (pentagonal shape, in this figure) by way of gate valves (not shown). In addition, the Ti sputtering chamber 204 can be used for forming a TiN film by reactive sputtering only by adding a nitrogen based atmosphere gas, so that it can used to continuously form a Ti film and a TiN film by changing the gas composition during the process. The wafer is carried in and from each chamber by a wafer carrying robot 207 provided in the carrying chamber 201. In this system, the wafer is exposed to atmospheric air only when carried from the load lock chamber 202. In other words, the wafer is carried in a state being shielded from atmospheric air between the other processes than carrying from the load lock chamber 202.
The multi-chamber system conceptually enables the above-described ideal processes; however, it requires a vast equipment investment, and is disadvantageous in making it impossible to make use of the existing sputtering system. The system, which is large in size, has another disadvantage in that it occupies a large floor space in an expensive clean room. As a result, the multi-chamber system tends to increase the manufacturing cost of semiconductor devices.
On the other hand, it may be considered that the underlying film is formed by CVD in place of sputtering for improving the coverage thereof on the side wall surface of a connection hole; however, in this case, a problem is encountered in growth of a metal oxide on the underlying film. Such a problem occurring in the case of using a Ti/TiN laminated film as the underlying film will be described with reference to FIGS. 19 and 20.
Referring to FIG. 19, a field oxide film 42 (SiO.sub.2) is formed on a Si substrate 41 for element isolation, an impurity diffusion area 43 being formed, and a SiO.sub.x interlayer insulating film 44 is formed. A contact hole 45 reaching the impurity diffusion area 43 is formed in the Sio.sub.x interlayer insulating film 44, and is then sequentially covered with a thin Ti film 46 and a thin TiN film 47 by CVD. The Ti film 46 reduces a natural oxide film on the surface of the Si substrate 41 and the film 46 itself is converted into a silicide, to achieve a low resistance ohmic contact. The TiN film 47 is provided for ensuring a barrier performance which is insufficient only by the Ti film 46, so that an Al--Cu film 49 (see FIG. 20) burying the contact hole 45 in the subsequent process is blocked from being penetrated in the Si substrate 41. The step coverage of each of the Ti film 46 and the TiN film 47 is excellent because it is formed by CVD. In particular, the thickness of the Ti film 46 on the side wall surface of the hole is made larger than that of the Ti film 34 shown in FIG. 15.
However, since the CVD system for forming the Ti film 46 and the TiN film 47 is independent from the sputtering system for forming an Al--Cu film 49 (see FIG. 20) in the subsequent process, the wafer must be exposed to atmospheric air once after completion of film formation by CVD. At this time, a Ti oxide film 48 is formed on the surface of the TiN film 47 as shown in FIG. 19.
When an Al--Cu film 49 is intended to be formed on the substrate and then subjected to high pressure reflow as shown in FIG. 20, an Al oxide film 50 is formed at the boundary between the TiN film 47 and the Al--Cu film 49 by oxygen diffused from the Ti oxide film 48 and it obstructs the thermal flow of the Al--Cu film 49. Accordingly, the burying of the contact hole 45 with the Al--Cu film 49 by high pressure reflow does not proceed over a specified level, as a result of which a void 51 remains. Such a problem may be solved using the multi-chamber system shown in FIG. 18 in which the CVD system is integrated with the sputtering system; however, the multi-chamber system has various physical and economical problems to be solved as described above.
As seen from FIGS. 16, 17 and 20, the Al oxide films 37, 38 and 50 finally obstruct the smooth burying of the connection hole by high pressure reflow. Specifically, the Al oxide film 37 (see FIG. 16) formed on the surface of the Al--Cu film 35 by direct oxidation due to contact with atmospheric air, and the Al oxide films 38 and 50 (see FIGS. 17 and 20) formed on the lower surfaces of the Al--Cu films 35 and 49 by secondary oxidation by way of the Ti oxide films 39 and 48, exert adverse effect on high pressure reflow.